2008A&A...483..933E

Atomic hydrogen escaping from the planet HD209458b provides the largest observational signature ever detected for an extrasolar planet atmosphere. However, the Space Telescope Imaging Spectrograph (STIS) used in previous observational studies is no longer available, whereas additional observations are still needed to better constrain the mechanisms subtending the evaporation process and to determine the evaporation state of other ``hot Jupiters''. Here, we aim to detect the extended hydrogen exosphere of HD209458b with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) and to find evidence of a hydrogen comet-like tail trailing the planet, whose size would depend on the escape rate and the amount of ionizing radiation emitted by the star. These observations also provide a benchmark for other transiting planets, in the frame of a comparative study of the evaporation state of close-in giant planets. Eight HST orbits were used to observe two transits of HD209458b. Transit light curves were obtained by performing photometry of the unresolved stellar Lyman-α (Lyα) emission line during both transits. Absorption signatures of exospheric hydrogen during the transit were compared to light curve models predicting a hydrogen tail. Transit depths of (9.6±7.0)% and (5.3±10.0)% were measured on the whole Lyα line in visits 1 and 2, respectively. Averaging data from both visits, we find an absorption depth of (8.0±5.7)%, in good agreement with previous studies. The extended size of the exosphere confirms that the planet is most likely losing hydrogen to space, yet, the photometric precision achieved does not allow us to better constrain the hydrogen mass-loss rate.